A double-flow turboshaft engine is known to comprise a central generator emitting a flow of hot gas and an annular bypass duct (fan channel) surrounding said central generator and through which passes a bypass flow of relatively cold gas coming from a fan. Such a turboshaft engine is equipped, more and more often, with a single nozzle which extends said annular duct outwardly beyond the outlet of said central generator and inside which the two flows, hot and cold, are freely confluent. In such a so-called confluent or compound nozzle, the hot flow from the central generator is surrounded by the cold bypass flow coming from the fan.
Since, within the confluent nozzle, there is no solid surface separating the two flows, the respective final sections of the flows at the outlet of said common confluent nozzle result not only from the geometry thereof, but also from the equilibrium of the pressures along the fluid surface (interface) separating the two flows. It will be readily understood that if, for example, the pressure of the cold bypass flow increases, the hot central flow will have to be compressed radially: in that case, the final annular section of the cold flow will thus increase, whilst the final central section of the hot flow will decrease.
For a given engine, these variations in relative sections will, of course, depend on the conditions of operation of said engine, such as speed and altitude of the aircraft and temperature of the ambient air, as well as on the position of the throttle lever and they do not coincide exactly with the optimum dimensions enabling the best possible performances of the engine to be obtained.
It will be noted that, for most uses, such relative variations in section of flow with respect to ideal sections giving the best performances are not too detrimental, as they are partly compensated by the versatility of the double-flow engines.
However, the maximum thrust developed by these engines generally corresponds either to the obtaining of a limiting operational temperature, or to a limiting speed of rotation (limiting power), these two limits being only exceptionally concomitant. Thus, in the particular, important, case of the thrust upon take-off at high ambient temperature, this thrust is currently defined by the limiting temperature, whilst the speed of rotation is clearly less than the limiting speed.
It is therefore the object of the present invention to provide, in simple manner, a variation in the geometry of a confluent nozzle, so that the operational conditions of the turboshaft engine are as close as possible to the ideal conditions.